1. Field
The disclosed embodiments refer to a safety device for a semiconductor switch, used specifically to open or close secondary lines, connected to a primary network and making it possible to obtain power from secondary equipment.
2. Brief Description of Related Developments
Semiconductor switches provide electrical protection making it possible to open the secondary circuits and to protect the secondary network cables in the event of over current in the secondary circuits.
The electrical protection is calibrated to open the switch at a certain calculated value for the i2·t current, lower than the smoke curve rating characterizing the cables that are specifically used in aircraft wiring.
These semiconductor switches are generally equipped with a protective electrical circuit, comprising a means to detect the current crossing the secondary line that measures the potential difference at the terminals of a resistance series inserted into the secondary circuit, and, comprising a control circuit for the switch component, measuring the value of the current in the secondary circuit line and activating the switch component in the open circuit in the event of overcurrent exceeding a current threshold defined in the secondary line.
The semiconductor switch is thus equipped with an open-close control command that is automated under normal operation by an external system and with an open command emitted from the protective electric circuit.
A problem encountered in these switches is that their failure mode is generally a short-circuit in the switch component which is usually constituted by a MOS power transistor.
When this type of failure occurs, it is no longer possible to control the secondary line opening which will remove the overcurrent protection and may lead to destructive overheating of the secondary cables in the event of problems with the equipment which might even result in damaging of the primary line.
A known solution is to place a fuse upstream from the semiconductor switch, this fuse being calibrated to cut-off when a threshold value higher than the safety cut-off value for the switch is reached.
The fact remains that in former art, the cut-off of this fuse occurs only in the event of failure of the downstream circuit and, because the characteristics of the fuses are dispersed, with the exception of when the fuse is configured to a rupture value that is much lower than the current capacity of the cable, it cannot be easily guaranteed that the circuit will open at a level of intensity that will not be prejudicial to the cables when the switch is short-circuiting.
Therefore, the cables must be oversized in comparison with the cut-off value of the fuse as well as with the cut-off current of the switch device and the switch device must be also oversized compared to the nominal current of the secondary line in order to reduce the risk of destroying the power transistor.